The present invention relates to a picture display device and more particularly to a picture display device using a liquid crystal.
FIG. 1 shows a conventional picture display device to display a picture comprising a liquid crystal and a column and row array of picture elements arranged in a matrix configuration. A unit picture element comprises a MOS-FET 1, a storage capacitor 2 and a liquid crystal cell 3. The fundamental operation of the unit picture element will be illustrated. When a negative voltage pulse is applied to a gate line X.sub.i as a gate signal, the MOS-FET 1 of a P channel is turned on and an analogue video signal applied to a signal line Y.sub.i charges the storage capacitor 2 through the FET 1. The FET 1 is turned off if the negative voltage pulse is removed. Since the lead currents of the FET 1 and the liquid crystal cell 3 are quite small, a voltage in proportion to the video signal charged in the capacitor 2 is maintained applied to the liquid crystal cell 3 for a considerably long time. The gate signals are sequentially scanned on the row electrode lines X.sub.i, X.sub.i+1, X.sub.i+2 . . . , and the video signals corresponding to the lines are simultaneously applied to the column electrode lines Y.sub.i, Y.sub.i+1, Y.sub.i+2, . . . , whereby the overall picture is diplayed.
FIG. 2 shows a sectional view of a picture element including the FET 1. In the P-channel MOS-FET, reference numeral 4 denotes an n type S.sub.i substrate, 5 and 6 denote respectively a source and a drain in a P.sup.+ diffusion region, and the source 5 is connected in the Y.sub.i direction in FIG. 1. Reference numeral 7 denotes a gate oxide, and 8 denotes a gate electrode connected in the X.sub.i direction. Reference numeral 9 denotes a picture electrode of a liquid crystal cell and the electrode 9 also forms of a capacitor together with a thin oxide 11 and the substrate 4. Reference numeral 12 denotes a liquid crystal, and 13 denotes a transparent counter electrode which serves as a common electrode for the overall picture area. Reference numeral 14 denotes a surface glass substrate and 10 denotes an insulation film. In this structure the voltage polarity applied to the liquid crystal is unidirectional and it is directly driven. Therefore the liquid crystal is of short life and of low reliability.
A method of inverting the video signals in synchronism with the signals applid to the common electrode at a given period of time has been considered to drive the liquid crystal alternately. If the video signal is inverted at a period of around 30 Hz, half of the picture is displayed negatively. Although the signal has been inverted at a period of 1 Hz, and 10 Hz or more to avoid the negative display, such is not practicable as the picture flickers with each inversion.
Accordingly, it is an object of the present invention to provide a picture display device of longer life and of greater reliability without disturbing or flickering the picture.